Color television signal reproducing system

ABSTRACT

A color television signal reproducing system in which a NTSC composite color television signal recovered from a magnetic recording and reproducing system such as a VTR and containing timing error information is processed to produce a continuous reference signal locked to timing variations in the horizontal sync signal which is separated from the composite signal. The composite signal is processed with the reference signal thus derived by means of balanced modulation to obtain a timing error corrected color subcarrier signal containing reduced timing error components.

United States Patent [1 1 [111 3,749,826

Arimura July 31, 1973 [5 COLOR TELEVISION SIGNAL 3,609,223 9/1971 Tajiril78/5.4 c1) REPRODUCING SYSTEM 3,614,305 l0/l97l Hidaka 178/5.4 CD

[7 51 Inventor: Ichiro Arimura, Kyoto, Japan Primary Examiner-RichardMurray [73] Assignee: Matsushlta Electric Industrial Co. A M h Ltd.Kadomebshi' Osaka, Japan orney Stevens, Davis, Ml er & 0s er [22] Filed:May 11, 1971 [21] Appl. No.: 142,329 [57] ABSTRACT A color televisionsignal reproducing system in which [30] Foreign Application Priority DamNTSC compssite colgr televijsion sigial recovetred rom a magne 1c recormg an repro ucmg sys em May 13, 1970 Japan 45/41239 such as a VTR andcontaining timing error information is processed to produce a continuousreference signal (gl. 213649751; locked to timing variations in thehorizontal Sync Signal [58] Fieid 5 5 4 which is separated from thecomposite signal. The comearc 8/5 4 5 posite signal is processed withthe reference signal thus derived by means of balanced modulation toobtain a timing error corrected color subcarrier signal contain- [56]References Cited ing reduced timing error components.

UNITED STATES PATENTS 3,629,491 12/1971 Dann 17815.4 CD 4 Claims, 4Drawing Figures DELAY LPF LINE MIXER 7) 36m; 9, IO, M

BPA BM BM 0.5M; 4140/: 27M 1 .SY/VC SEP F/XEO MO/VOMW/ 056 WLT/PL/ER50M? 316MHz /2 5 EM BM i EPA x P- +5.0,

FIXED AFC COLOR TELEVISION SIGNAL REPRODUCING SYSTEM This inventionrelates to the removal of timing irregularities or variations introducedinto the NTSC composite color television signal in the course ofrecording and reproducing thereof by a recording and reproducing systemsuch as a VTR either directly or by the low frequency shifting of thecolor signal component thereof.

More specifically, the invention concerns the partial or entirecancellation of timing errors in the recovered color signal byprocessing the color signal in the presence of a frequency multipliedreference signal, which is produced from the separated horizontal syncsignal and is phase locked to the timing variations in the sync signal.

The invention is useful where reproduction of the color signal withfidelity by the recording and reproducing system is impossible due totiming errors introduced thereby.

A method of cancelling timing errors in the recovered carrier wave isdisclosed in the U.S. Pat. No. 3,095,427 (issued to Ampex Corporation).In this method, in the recording of a composite color television signala reference signal (pilot signal) of a frequency lying outside thecomposite color television signal frequency modulation band issimultaneously recorded and superimposed upon the composite signal andthe reproduced color signal and reference signal are processed in thepresence of a stable local oscillator by a frequency conversiontechnique involving the addition and subtraction of these three signals,thereby removing timing error components from the reproduced colorsignal.

The U.S. Pat. No. 2,979,588 (issued to Radio Corpo ration of America)disclosed another method of removing the timing errors from thereproduced color signal. In this method, a reference signal locked totiming variations in the burst signal in the recovered composite colortelevision signal is produced and the recovered color signal isprocessed with the reference signal in the presence of a stable fixedfrequency source to thereby remove the timing errors.

In the above prior-art methods, however, certain drawbacks are present.In the former method, the pilot signal must be separately provided andthe extraction of the pilot signal involves some difficulties. In thelatter method, in the production of the continuous reference signal fromthe burst signal it is difficult to achieve amplitude and frequencyresponse to the timing variations with fidelity if known techniques suchas a linking oscillator or APC (automatic phase control) are employed.

The principal object of the invention is to provide a method of and asystem for carrying out the removal of part or all of the timing errorsin the color signal component in a recovered composite color televisionsignal containing timing error information.

According to the invention, a frequency multiplied reference signallocked to timing variations in the horizontal sync signal, which isseparated from the recovered composite color television signal, isproduced from the separated horizontal sync signal and the color signalcomponent in the recovered composite color television signal and thereference signal are processed in the presence of a stable fixedoscillator by using known additive and subtractive frequency conversiontechniques to thereby cancel part or all of the timing errors in thecolor signal component.

The above and other objects, features and advantages of the inventionwill best be understood from the following description, when read inconnection with the accompanying drawings, in which:

FIG. 1 illustrates the principles underlying the invention;

FIG. 2 is a block diagram showing an example of the method of producingfrequency multiplied reference signals from the separated horizontalsync signal;

FIG. 3 is a block diagram showing an example of the practicalapplication of the principles underlying the invention when cancellingtiming errors in the chromi nance signal component in the recoveredcomposite color television signal; and

FIG. 4 is a block diagram showing another example of the application ofthe principles underlying the invention.

FIG. 1 shows the principles underlying the invention. According to theinvention, the recovered color signal of center frequency jg includingtiming error information and the horizontal sync signal at frequency falso including timing error information, as represented by a in FIG. 1,are processed to produce a reference signal with frequency f asrepresented by b, which is locked to phase variations in the horizontalsync signal f The reference signal thus obtained is used, for example,to derive the f color signal, a frequency converted color signal havinga center frequency f in which the timing error component is partially ortotally cancelled out.

To this end, a stable signal source tuned to a fixed freshest; f; (f:being made @4031 to f; f +fgi, as represented by c. is provided. Thisj}, signal and the f color signal are additively combined to produce afrequency converted color signal having a center frequency (f f a asrepresented at d. This sum combination signal is then combined with theabove noted ta-arenas aghast the frequencyf'l whose timing variationsare substantially the same as those of the sum signal, thus obtaining asecond frequency converted color signal having a center frequency f B (Adotted signal such as f represents the signal which contains a timingerror component.)

It is to be appreciated that the timing error component in the resultantf 3 color signal may be reduced or eliminated. If f f perfectcancellation of the timing error component is theoretically possiblesince the "fi ming vafiatiahsrntraauea -ifitii tlieifli "s'ign'aifaithfully follow those in the f signal. Further, if f H is not equal tof the timing error component introduced into the frequency signalcomponent of the frequency difference f}, -f,, remains throughout theprocess mentioned above. However, where the frequency difference f f,,is not so great, the remaining timing error component contained in thefrequency signal f f is as small as (f -f',,,)/f times the timing errorcomponent of the frequency signal f and thus the remaining timing errorcomponent can be reduced considerably.

Thus, it is advantageous that in the recording and reproduction of theNTSC composite color television signal the color subcarrier frequency isshifted to a frequency as low as possible. By so doing, the absolutemagnitude of timing variations introduced by the recording andreproducing apparatus such as a VTR may be minimized.

FIG. 2 shows an example of the method of producing the above notedreference signal, which is a frequency multiplied signal locked in phaseto the timing variations in the horizontal sync signal. Referring toFIG. 2, a recovered composite color television signal is fed to terminal1 and the sync signal is separated by a sync signal separator 2. Theseparated sync signal is differentiated to drive a 50-50 duty ratiomonostable multivibrator 3, whose rectangular wave output is fed to awell-known frequency multiplier 4, which in turn provides an output asthe reference signal at a desired frequency of m/n times f available atoutput terminal 5.

The frequency multiplier 4 may be constructed by using well knowntechniques of frequency multiplication employing a successive connectionof harmonic band-pass amplifier, distorted wave generator, bandpassamplifier and so on.

FIG. 3 shows an example of the system for cancelling the timing errorsor irregularities present in the color signal in a composite colortelevision signal reproduced from, for instance, a VTR.

The reproduced composite color television signal appearing at an inputterminal 6 is coupled to a bandpass amplifier 7, which separates thecolor subcarrier signal (3.6 i 0.5 MHz). The input composite signal isalso coupled to a circuit 8, which has the same functions as describedabove in connection with FIG. 2 and produces an output (referencesignal) at a frequency nf phase locked to timing variations in thehorizontal sync signal. In this example,

This 2.7-MHz reference signal and the color subcarrier signal output ofthe band-pass amplifier 7 are coupled to a balanced modulator 9, forbalanced modulation, to produce a frequency shifted color signal (0.9 t0.5 MHz).

It is to be noted that the timing error in the low center frequencycolor signal thus obtained is 0.9/3.6 1/4 of the corresponding timingerror in the 3.6-MHz color subcarrier signal separated from therecovered composite color television signal.

The 0.9-MHZ color signal is coupled to another balanced modulator 10, towhich is connected a stable fixed oscillator 11 oscillating at afrequency of 4.1 MHz, and which produces a high frequency shifted colorsignal (5.0 i- 0.5 MHz) having a sum frequency of 5.0 MHz as the centerfrequency. In this high frequency shifted color signal, the timing erroris further reduced by a considerable extent. Thus, this signal can beprocessed for the cancellation of residual timing errors by well-knownmethods such as a timing error cancelling method shown in FIGS. la andlb of the U.S. Pat. specification No. 2,979,558 issued to RadioCorporation of America and a timing error cancelling method shown inFIG. 2 of the U.S. Pat. Specification No. 2,921,976.

In the present example, this 5.0-MHz color signal is coupled to afurther balanced modulator 12, to which is connected a stable fixedoscillator 13 oscillating at a frequency of 3.6 MHz, producing a furtherfrequency shifted color signal having a still higher center frequency of8.6 MHz. The 5.0-MHz color signal is also coupled to an APC 14, toproduce a continuous signal synchronized to the color burst (5.0 MHz)and following the residual timing errors. The 8.6-MHZ color signal andthe 5.0-MHz continuous signal are coupled to another balanced modulator15, for balanced modulation, to produce a low frequency shifted colorsignal (3.6 i 0.5 MHz) having a difference frequency of 3.6 MHz as thecenter frequency. In this resultant 3.6-MHz low frequency shifted colorsignal the timing error components are completely cancelled. This colorsignal is fed to a band-pass amplifier 16 to filter out undesirablecomponents. The color signal output of the bandpass amplifier 16 freefrom undesirable components is mixed in a mixer 19 with the luminancesignal output of a LPF 17 free from the chrominance signal componentderived from the recovered composite color television signal. Thus, themixer 19 provides a recovered composite color television signal outputin which the chrominance signal timing errors are cancelled. A delayline 18 is inserted to compensate for the time difference between theluminance signal processing circuit and the chrominance signalprocessing circuit. The APC 14 is of a well-known construction found inusual television receiving sets, producing a continuous output locked infrequency and phase to the intermittent burst input.

In the preceding embodiment, timing errors in part of the color videosignal are cancelled in accordance with the present invention, and theremainder of the introduced timing variations are completely removed bywell-known color television signal reproducing techniques. Thechrominance signal timing error compensated composite color televisionsignal derived in the above manner can reduce the difficulty in theconventional color television signal reproducing technique in followingthe APC burst output, thus providing equivalent effects of stabilizingthe APC operation. In the preceding embodiment, the 4.1-MI-Iz fixedoscillator is used to produce the 5.0-MHz color signal output for thesole purpose of preventing harmonic components from being introduced inthe color signal output of the succeeding stage balanced modulator.

FIG. 4 shows an example of completely cancelling the timing variationsin the recovered color signal. In this example, a recovered, lowfrequency shifted color signal containing timing error information isprocessed for timing error cancellation to provide a regular colorsignal.

The low frequency shifted color signal reproduced from a VTR isimpressed on an input terminal 21. In this example, it is assumed thatthe center frequency of the input signal is 688 kHz, which is 5/(2 X 13)times the standard color subcarrier frequency f 3.58 MHz. The frequencyf is expressed in terms of the horizontal sync frequency as A circuit22, which has the same functions as described in connection with FIG. 2,produces a frequency multiplied, reference signal of a frequency equalto the center frequency 688 kHz of the recovered input color signal,that is, a frequency equal to (S X 7 X 5 )/4 times the horizontal syncfrequency f The 688-kHz reference signal from the circuit 22 is coupledto a balanced modulator 24, to which is connected a stable fixedoscillator 23 oscillating at a frequency of 3.58 MHz, and which producesa 2.89-MHz output containing timing variations in the opposite sense.This output signal and the 688-kHz color signal are combined by abalanced modulator 25 to produce a 3.58-MH2 NTSC color signal. In thebalanced modulator 25, the timing error is completely cancelled, sincethe timing error in the 688-kHz signal is the same in magnitude as andopposite in sense to that in the 2.89-MHZ signal.

In the foregoing, the principles underlying the invention and someembodiments of the invention have been described. It will be apparent,however, that various other changes and modifications in theorganization and mode of operation are possible without departing fromthe scope and spirit of the invention.

Since it is important to effectively cancel the timing variationsintroduced in the recording and reproduction of television signals froma VTR and other means in order to stabilize particularly the display ofthe reproduced signals, it is to be seen that the invention providessolutions to various technical difficulties encoun' tered in televisionsignal recording and reproducing systems.

What is claimed is:

l. A color television signal reproducing system for obtaining afrequency shifted color signal containing a reduced timing errorcomponent from a recovered composite color television signal includingtiming error information comprising: i

a. first means for separating the horizontal synchronization signal fromsaid recovered composite color television signal;

b. means, connected to said separating means, for

multiplying said separated horizontal synchronization signal to providea frequency multiplied reference signal having a predetermined frequencyand phase locked to timing variations in said horizontal synchronizationsignal;

c. second means for separating the color signal from said recoveredcomposite color television signal; and

(1. first means for combining said reference signal and said separatedcolor signal to produce said frequency shifted color signal containingareduced timing error component.

2. A color television signal reproducing system according to claim 1wherein said frequency shifted color signal produced by said firstcombining means represents the difference between said reference signaland said separated color signal.

3. A color television signal reproducing system according to claim 1further comprising:

a. means for providing a stable frequency source signal at apredetermined frequency; and

b. second means for combining said frequency shifted color signal andsaid stable frequency source signal to produce a sum frequency signal.

4. A color television signal reproducing system according to claim 1,which further comprises means for producing a continuous signal at adesired frequency and phase locked to timing variations in a burstsignal extracted from said frequency shifted color signal containing areduced timing error component.

i k l 9

1. A color television signal reproducing system for obtaining afrequency shifted color signal containing a reduced timing errorcomponent from a recovered composite color television signal includingtiming error information comprising: a. first means for separating thehorizontal synchronization signal from said recovered composite colortelevision signal; b. means, connected to said separating means, formultiplying said separated horizontal synchronization signal to providea frequency multiplied reference signal having a predetermined frequencyand phase locked to timing variations in said horizontal synchronizationsignal; c. second means for separating the color signal from saidrecovered composite color television signal; and d. first means forcombining said reference signal and said separated color signal toproduce said frequency shifted color signal containing a reduced timingerror component.
 2. A color television signal reproducing systemaccording to claim 1 wherein said frequency shifted color signalproduced by said first combining means represents the difference betweensaid reference signal and said separated color signal.
 3. A colortelevision signal reproducing system according to claim 1 furthercomprising: a. means for providing a stable frequency source signal at apredetermined frequency; and b. second means for combining saidfrequency shifted color signal and said stable frequency source signalto produce a sum frequency signal.
 4. A color television signalreproducing system according to claim 1, which further comprises meansfor producing a continuous signal at a desired frequency and phaselocked to timing variations in a burst signal extracted from saidfrequency shifted color signal containing a reduced timing errorcomponent.